A. Field of the Invention
The present invention relates to a basic structure element forming a magnetic sensor or a magnetic memory, and a device using the element. More specifically, the invention relates to an element that controls electron spin injection to form a magnetic random access memory having a large capacity and including no mechanical driving section, or to an element forming a weak electric current sensor that detects electron spin injection, and a device using the element.
B. Description of the Related Art
FIG. 8 is a view schematically showing a cross sectional structure for explaining an example of an arrangement of a previously proposed GMR (Giant Magneto-Resistance) element making use of a GMR effect. For example, on silicon insulation substrate 200 with a silicon oxide film formed on the surface thereof, fixed layer electrode 208, ferromagnetic fixed layer 205 (with a thickness of approximately 40 nm and a diameter of approximately 200 nm) made of a material such as Co, isolation layer 204 (with a thickness of approximately 6 nm and a diameter of approximately 200 nm) of nonmagnetic metal, ferromagnetic free layer 206 (with a thickness of approximately 2.5 nm and a diameter of approximately 200 nm) made of a material such as Co, and free layer electrode 207 are formed in that order. It is known that such a GMR structure element can reverse the direction of magnetization of ferromagnetic free layer 206 by spin current injection from the side of free layer electrode 207, that is, injection of electrons with polarized spins from the side of fixed layer electrode 208 (see, for example, JP-A-2004-207707 and J. A. Katine et al., “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars”, Physical Review Letters, Vol. 84, No. 14, pp. 3149-3152 (2000)).
The operation principle of the element is explained as follows. First, a magnetic field with a sufficient strength is applied to the element to align the directions of magnetization of ferromagnetic fixed layer 205 and ferromagnetic free layer 206 in the same direction. FIG. 9A is a schematic cross sectional view showing the case in which the directions of magnetization in the ferromagnetic layers in the element shown in FIG. 8 are aligned rightward, with arrows in the figure showing the directions of magnetization in the respective ferromagnetic layers. This state is referred to as the parallel state (P-state). In this state, an electric current made to flow in the direction from the side of fixed layer electrode 208 to the side of free layer electrode 207 causes electrons to be injected from free layer 207 into ferromagnetic free layer 206. In free layer electrode 207, the electron spins are in a state in which the distribution of up-spins matches that of down-spins. In ferromagnetic free layer 206, however, due to interaction (s-d interaction) between the electron spins and the spins of ferromagnetic metal atoms, spins of the conduction electrons are polarized so that the spins in parallel with the direction of magnetization of ferromagnetic free layer 206 are a majority. This is referred to as polarization of spin. However, ferromagnetic free layer 206 of the layered films now being considered is thin, so that the polarization is slight. When the conduction electrons with their slightly polarized spins pass through isolation layer 204 to reach the surface of ferromagnetic fixed layer 205, electrons having spins in parallel with the direction of magnetization of ferromagnetic fixed layer 205 are injected into ferromagnetic fixed layer 205. However, electrons having spins that are opposite to the direction of the magnetization of ferromagnetic fixed layer 205 are reflected to be injected into ferromagnetic free layer 206 again. Ferromagnetic fixed layer 205, being thick, functions as a spin filter. It gives priority to the passage of electrons that have spins that are in parallel with the direction of the magnetization of ferromagnetic fixed layer 205. As a result, majority carriers in ferromagnetic free layer 206 become electrons that have spins opposite to the direction of the magnetization of ferromagnetic fixed layer 205. Each of the electrons exerts a torque on the magnetization of ferromagnetic free layer 206 in the direction to reverse the direction of magnetization thereof. A current exceeding a certain level of a critical current causes the direction of magnetization of ferromagnetic free layer 206 to rotate by the exerted torque, by which the state with the directions of magnetization of ferromagnetic free layer 206 and ferromagnetic fixed layer 205 changes from the P-state shown in FIG. 9A to an anti-parallel state (AP state) shown in FIG. 9B.
Next, an explanation will be made about the case in which a current is made to flow from free layer electrode 207 to fixed layer electrode 208 in the element in the AP-state. In this case, electrons are injected from fixed layer electrode 208 to ferromagnetic fixed layer 205. Also in fixed layer electrode 208, the electron spins are in a state in which the distribution of up-spins matches that of down-spins. In the ferromagnetic layers, however, there is interaction (s-d interaction) between the electron spins and the spins of ferromagnetic metal atoms. Here, thick ferromagnetic fixed layer 205 causes spins of the conduction electrons to be polarized so that spins in parallel with the direction of magnetization of ferromagnetic fixed layer 205 are a majority. When the conduction electrons with largely polarized spins pass through isolation layer 204 to reach the surface of ferromagnetic free layer 206, a majority of electrons having spins in antiparallel with the direction of magnetization of ferromagnetic free layer 206 are injected into ferromagnetic free layer 206. As a result, each of the injected electrons which have spins in a direction that is in parallel with the direction of the magnetization of ferromagnetic fixed layer 205 exerts a torque on the magnetization of ferromagnetic free layer 206 in the direction to reverse the direction of magnetization thereof. A current exceeding a certain level of a critical current causes the direction of the magnetization of ferromagnetic free layer 206 to rotate by the exerted torque, by which the state with the directions of magnetization of ferromagnetic free layer 206 and ferromagnetic fixed layer 205 returns from the AP-state shown in FIG. 9B to the P state shown in FIG. 9A.
The electric resistance between two electrodes of a GMR element is known to be small in the P-state and large in the AP-state with the rate of change being several tens of percent. By using the GMR effect, a memory element can be formed. However, for causing magnetization reversal by current injection, a large current density of the order of 108 A/cm2 is necessary at present.
The above-explained technology causes magnetization to be inverted by flowing a current in the element. Its operation principle is that magnetization reversal is caused by a torque exerted on the magnetization of the ferromagnet due to electron spin when spin-polarized electrons are injected into a ferromagnet. Therefore, in order to lower the current density necessary to cause magnetization reversal, the magnetization reversal must be caused by a slight amount of injected electrons. This necessitates that the volume and the magnitude of saturation magnetization of a ferromagnetic free layer to be subjected to magnetization reversal be made small. However, making the volume and the magnitude of saturation magnetization of a ferromagnetic free layer small results in degradation of resistance of recorded magnetization to thermal agitation, and this makes minute magnetization unstable. As a result, a problem arises which makes retention of magnetization impossible.
The present invention was made in light of the foregoing problem with an object of providing a memory element and a weak current sensor in which a characteristic of retaining recorded magnetization and facility in magnetization reversal that enables magnetization reversal with a small current density are made compatible.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.